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The other day I was talking with a friend (yes, I have friends) about the way communication of ideas has changed. The Internet is the obvious advance, but what used to happen when an important new idea needed to be disseminated? As an example of how things used to be, I told him the story of Bohr's famous train trip.

In 1925, two students at the University of Leiden, Sem Goudsmit and George Uhlenbeck, realized that the fourth electron quantum number could be explained if the electron had spin. This was a radical idea (a point particle spinning?), and coming from students was doubly suspect. Physicists throughout Europe were excited yet skeptical. When Bohr was planning a trip from Cophenhagen to Leiden for a conference, it seemed an excellent opportunity to talk to the students to help understand if they were right.

In his book, Inward Bound, Abraham Païs narrates the story as told to him by Bohr twenty years later: